A typical coating system disposed over a substrate includes some or all of the following layers: (1) one or more primer layers that provide adhesion and basic protection, and also cover minor surface unevenness of the substrate; (2) one or more basecoat layers, typically pigmented, that provide durability and color; and (3) one or more clearcoat layers that provide additional durability and improved appearance. A colored topcoat layer can be used in place of the basecoat layer and clearcoat layer.
In some industrial applications, such as coating metal pipes, trucks, and other industrial equipment, it is often desired to complete the coating process in a short period of time while still achieving good adhesion, protection, durability and appearance. Conventional coating compositions typically produce a thin dried and cured coating layer that may not have sufficient thickness to cover unevenness of the substrate if only a single layer is used. Coating unevenness could result in undesired appearance. When conventional coatings are applied at a high coating thickness, surface coating defects such as microfoaming and low gloss may occur. In addition, thick coating layers can be prone to sagging defects, especially for coating layers applied on vertical surfaces.
Because of the known sagging problems when applying coating compositions, sag control compositions are often added to form a smooth sag-free surface. Typical sag control compositions are produced by reacting an amine-containing compound with an isocyanate-containing compound in the presence of a coating resin such as, for example, an acrylic resin. The resulting sag control compositions include urea particles that contain the reaction product of the amine-containing compound and the isocyanate-containing compound as well as the coating resin.
However, such sag control compositions present a number of challenges. Conventional sag control compositions are specific to the particular coating compositions that include the coating resins used to form the sag control compositions, and facilities that employ coating compositions with different coating resins must generally employ sag control compositions that are specific to each different coating composition. Further, such sag control compositions also generally require special offsite compounding by external suppliers to form the sag control compositions, thereby increasing cost and production inefficiency.
In addition, some sag control compositions are prepared by reacting an amine-containing compound with an isocyanate-containing compound in the presence of various solvents. However, due to viscosity considerations and the impact of high viscosity on ease of use, the sag control compositions are generally prepared at low concentrations of less than about 4 weight % of the urea particles based on the total weight of the sag control compositions. Such sag control compositions exhibit poor stability, with the urea particles quickly separating out of solution, and are therefore not employed in conventional coating systems.
Accordingly, it is desirable to provide sag control compositions that can be employed in various coating compositions that contain different coating resins and that exhibit excellent stability. In addition, it is desirable to provide methods of forming such sag control compositions. In addition, it is desirable to provide methods of forming coating systems that include at least one coating layer including urea crystals of such sag control compositions. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.